The present invention relates to a method of producing shaped articles made from single-sidedly or double-sidedly galvanized steel sheet, starting from galvanized steel strip, at least one of the steps of the method being a transport operation, and in which, for protection from black-spot corrosion, a corrosion preventive oil is applied which comprises at least one phosphoric acid polyoxyalkylene ester.
The production of flat metallic ready-made products from galvanized steel, such as automobile bodies or parts thereof, appliance casings, exterior architectural facings, ceiling panels or window profiles, for example, is a multi-stage operation. The raw materials for it are usually galvanized steel strips which are produced by rolling of the metal, followed by galvanizing, and which for storage and transportation are wound to form rolls (referred to as coils). For processing, these coils are wound again, separated into smaller pieces, and shaped by means of suitable techniques such as punching, drilling, folding, profiling and/or deep-drawing. Larger components, such as automobile bodies, for example, are optionally obtained by the joining of two or more individual parts. After shaping and joining have taken place, the product can be painted, for example.
A characteristic of the stated production operation is that not all of the steps referred to are performed in one manufacturing site; instead, as a general rule, precursor products and/or semifinished products must be transported one or more times from one manufacturing site to another. To take as an example the production of automobiles: the production of the metal strips takes place at the premises of a steelmaker. The cutting-up of the strips and the shaping to an automobile body or bodywork parts takes place in a pressing plant, and the manufactured bodies or parts thereof are then transported to an automaker for painting and final assembly.
Also deserving of mention in this context is the “completely knocked down” or “partly knocked down” manufacturing technique for automobiles, where vehicles intended for export are transported deliberately not in the fully assembled state but instead in the form of individual parts to the importing country, and undergo final assembly only in that importing country. With this manufacturing technique, entire bodies or bodywork parts must be transported form the exporting to the importing country, in some instances in ocean voyages that can take several weeks.
In the course of transport, on railroad wagons or in ships, for example, the precursor products and/or semifinished products are subject to atmospheric influences, and must therefore be protected from corrosion for their transport.
For corrosion protection in transport, it is common to apply what is called a “temporary protective”; in other words, this is not yet the final corrosion preventive coating, which is intended to impart permanent protection to the finished product, but is instead a coating which is removed at a later point in the process and replaced by the ultimate corrosion preventive coating. For temporary protection from corrosion, the steel strips are provided generally with a coating of a corrosion, preventive oil. Corrosion preventive oils often have a dual function and also act as forming auxiliaries, as during deep-drawing, for example. The forming oil is intended to ensure the necessary lubricity during the shaping operation, so as to prevent fracture or rupture of the metal sheet.
In the transport of shaped articles made from galvanized steel, one specific form of corrosion comes to the fore, namely that known as black-spot corrosion. This is a locally confined, rather than an extensive, form of corrosion. One possible cause of this black-spot corrosion is the possibility of contamination of the metal surfaces by particles in the course of transport. This particulate contamination then leads frequently to very locally confined forms of corrosion around the particles. The particles in question may for example be particles of dirt and/or of salt, or may be particles of salt in association with dirt.
Especially in the case of electrolytically galvanized steel, this form of corrosion also results in a significant change in the surface morphology. Viewed from the side, the metal surface is seen to have, for example, craterlike elevations. In the context of automobile construction, craterlike elevations of this kind are extremely disruptive, since they tend to be exacerbated, and certainly not leveled out, by the subsequent cationic deposition coating process. As a result of the black-spot corrosion, extremely extensive afterwork is necessary on the assembled body. This afterwork not only leads to high costs for the automaker but also disrupts the time course of the line manufacturing operation. Furthermore, the corrosion resistance of the completed body is adversely affected as well, since remediated spots constitute nucleation cells for the corrosion of the consumer product.
The use of phosphoric esters with alkoxy groups as corrosion inhibitors is known.
DE 27 56 747 A1 discloses the use of phosphoric esters, obtainable by reaction of phosphoric acids with alkoxylated polyols, such as polypropylene glycol, as low-foam corrosion-control and lubricant compositions.
U.S. Pat. No. 4,360,474 discloses derivatives of polyphosphoric monoesters and also their use as corrosion inhibitors, the ester groups being polyalkylene groups.
U.S. Pat. No. 4,684,475 discloses a radiator protection mixture which besides other components comprises an organophosphate comprising alkylene oxides as a corrosion inhibitor.
WO 00/42135 discloses the machining of metals using metalworking fluids which comprise phosphoric esters comprising oxyalkylene groups.
U.S. Pat. No. 5,555,756 discloses a method for improving the stretchability of a steel strip. For this method, the steel strip is first heated and then a liquid lubricant is applied to the surface and is subsequently dried, forming a dry film on the surface. The quantity applied is at least 10.8 mg/m2. The steel strip is subsequently rolled. The liquid lubricant comprises preferably water, a surfactant, and an alkyl phosphate ester of the general formulae RO—P(═O)(OH)2 or (RO)2—P(═O)OH, with R being an alkyl group having 4 to 20 carbon atoms. Phosphoric esters formed from alkoxylated alcohols are not disclosed.
None of the stated specifications, however, is concerned with the problem of black-spot corrosion in the transport of precursor products or semifinished or finished products of galvanized steel in an atmospheric environment.